Bad grounds are the biggest culprit. Are you sure the gauges are stock to the car? If someone just replaced the idiot light pod with the gauge pod, then the wiring pinouts may be different. In any case, I think you need to take the wiring diagram and verify that all the wires are in their correct positions in the harness connector for gauges. And of course the gauges may be faulty after 55 years. The FSM has procedures for testing them. I know when I swapped in full gauges on second gen cars a lot of re-pinning was required so the wiring was correct. At least on second gen cars the gauges were pretty common so replacing bad ones with swap meet finds was a simple effort.